Powered parking brake systems may utilize stored energy to apply the brakes upon command from a vehicle operator. These parking brake systems may be electrically or mechanically actuated. A majority of powered parking brake systems utilize a compressed coil spring as the energy source. This spring is usually held in the energized state by a pressurized piston. Another energy source is a charged hydraulic accumulator wherein hydraulic fluid stored under pressure is utilized for a parking brake application. When applying the parking brake, the fluid is used to move a piston which applies the parking load, either directly or indirectly. This may be an integral component of the service brake system or a separate parking assembly. When a hydraulic accumulator is employed, the final pressure applied is dependent upon the initial accumulator pressure, the stiffness of the braking system, and the volumetric capacity of the accumulator. The operating clearances and lightly loaded segments of the brake system consume the initial delivered volume at the highest pressures of the accumulator. The accumulator pressure decreases as fluid displaces the piston until the hydraulic system is in balance with the mechanical load which is at a maximum at this point in time. If additional brake load is desired for the system, the general practice is to increase the volumetric capacity of the accumulator so that the discharged or delivered volume of the accumulator is a smaller percentage of the total, and thereby resulting in a higher final pressure and thus higher braking load when fluid flow ceases.
It is desirable to provide a hydraulic actuation system for hydraulically powered parking brakes such as disclosed is copending U.S. Ser. No. 07/704,586 now U.S. Pat. No. 5,161,650 entitled "Disc Brake with Powered Integral Parking Mechanism" and assigned to the same assignee as herein. It is desirable to provide a system which effects a higher resultant final system pressure and, therefore, a higher mechanical brake load than is typically achieved in a system. It is advantageous if the system can deliver a higher final pressure with the same initial volume of fluid displacement. Thus, the system would be more efficient.